Reciprocating engines have machined crankshafts that rotate at high speeds, and also have pistons and connecting rods that oscillate up and down with every revolution of the machined crankshaft. These parts are referred to as the engine's rotating assembly. Machined crankshafts need to be highly engineered, so as to maximize the efficiency of the energy conversion process and also to resist shock loading issues and fatigue failure. Further, machined crankshafts need to be light and small, so as to provide fast response times and to be compact enough to fit inside of an engine block.
The magnitude of the force generated by an unbalance in any given rotating part depends on two things: first, the revolutions per minute (“RPM”) of the unbalanced rotating engine part, and second, the level of unbalance. The larger and heavier the part and the faster it rotates, the greater the forces generated by any unbalance. With respect to a rotating machined crankshaft, the force at the main bearings is proportional to the speed of the engine squared. And, additionally, the further the weight creating the unbalance is located from the center of gravity, the greater its effect on the rotating part as the rotating assembly spins. Typically, large heavy counterweights are used to offset the forces generated by the reciprocating weight of the pistons and rods. The machined crankshaft must not only maintain its own balance as it rotates inside the block, but it must also offset the forces generated by the mass of the pistons and rods as they pump up and down.
As just one example, crankshafts may be made by a deformation process in which the work is compressed between a pair of dies, using either impact or gradual pressure to form the part. Over time, the dies change and cause latent changes between respective crankshafts. As just one example, when the dies form the crankshaft, there may be a thin web left thereon where the two dies meet. The size of the thin web varies between the crankshafts, as its size is related to how much wear there is on the dies.
A crankshaft may be pre-balanced by spinning it to check for unbalanced conditions. Pre-balancing—or sometimes referred to as rough balancing—is based on a measured unbalanced condition, wherein first and second blind apertures are drilled into both ends of the crankshaft, so as to establish a machining axis for the crankshaft for later broaching, drilling, and grinding processes. However, known manufacturing processes, such as spinning processes, are unable to indicate exactly how the mass of the crankshaft is distributed. In particular, at least some known manufacturing methods are unable to account for the die wear and the associated variations manufactured into each crankshaft (as a result of each crankshaft being unique).
The proper placement of the machining axis may be critical to both pre-balancing the forged crankshaft and also end-balancing the machined crankshaft. In some cases, if the machining axis is improperly placed, then the machining processes may take too much material off of the forged crankshaft in one or more given places. The final, machined crankshaft may still have certain un-machined portions (e.g., the counterweights). The un-machined portions may affect the balance of the final, machined crankshaft and, in some case, so much so that the machined crankshaft cannot be properly balanced and is, thus, scrapped.